Apparatus for facilitating proximity communication between chips

ABSTRACT

One embodiment of the present invention provides a system for facilitating proximity communication between semiconductor chips. The system includes a base chip and a bridge chip, each of which includes an active face upon which active circuitry and signal pads reside, and a back face opposite the active face. The active face of the bridge chip is bonded to the active face of the base chip. Then, an identified portion of the active face of the bridge chip is thinned via etching and is removed by planarizing the back face of the bridge chip, thereby creating an opening in the bridge chip that exposes a portion of the active face of the base chip.

RELATED APPLICATIONS

This application is a divisional application of, and hereby claimspriority under 35 U.S.C. §120 to, application Ser. No. 11/385,430,titled “Methods and Structures for Facilitating ProximityCommunication,” by inventors A. Krishnamoorthy and J. Cunningham, filed20 Mar. 2006.

GOVERNMENT LICENSE RIGHTS

This invention was made with United States Government support underContract No. NBCH3039002 awarded by the Defense Advanced ResearchProjects Administration. The United States Government has certain rightsin the invention.

BACKGROUND

1. Field of the Invention

The present invention generally relates to semiconductor integratedcircuits. More specifically, the present invention relates to methodsand structures for facilitating proximity communication betweensemiconductor chips.

2. Related Art

Conductive electrical interconnections and transceivers facilitatereliable communications between integrated circuit (IC) chips. Becauseof packaging and manufacturing advantages, conductive interconnectionstypically dominate the interconnect hierarchy within computer systems.However, decreasing semiconductor line-widths and increasing on-chipclock speeds are putting pressure on the ability of traditionalresistive wires to achieve the off-chip bandwidths necessary to fullyutilize on-chip computational resources.

A new technique referred to as “proximity communication” overcomes thelimitations of resistive wires by using capacitive coupling to providecommunications between chips that are oriented face-to-face. Thiscapacitive coupling can provide signal densities two orders of magnitudedenser than traditional off-chip communication using wire-bonding ortraditional ball-bonding, while the circuits and coupling structuresremain fully-compatible with standard CMOS foundries. To communicateoff-chip through capacitive coupling, on-chip circuits drive ahigh-impedance, capacitive transmitter pad. Such communication avoidsimpedance conversion and thereby reduces the power normally dissipatedby off-chip driver circuits. Moreover, simple driver circuits and smallchip-to-chip distances can significantly reduce the total chip-to-chipcommunication latency.

While proximity communication provides off-chip signaling bandwidth thatscales with chip feature size, it also introduces topologicalconstraints. The active sides of chips typically need to face each otherwith full or partial overlap, so that corresponding transmitter andreceiver pads on opposing chips align both laterally and vertically.Since the strength of the capacitively-coupled signal voltage on areceiver pad is inversely proportional to the distance between thereceiver pad and a corresponding transmitter pad, maintaining a minimalvertical “z-separation” is important for successful communication.However, achieving and maintaining such alignment and proximity isdifficult, especially when working with rigid, noncompliant chips thattypically experience large temperature variations during operation.

Hence, what is needed are structures and methods that facilitateinter-chip alignment for proximity communication without the limitationsof existing approaches.

SUMMARY

One embodiment of the present invention provides a system forfacilitating proximity communication between semiconductor chips. Thesystem includes a base chip and a bridge chip, each of which includes anactive face upon which active circuitry and signal pads reside, and aback face opposite the active face. The active face of the bridge chipis bonded to the active face of the base chip. Then, an identifiedportion of the active face of the bridge chip is thinned via etching andis removed by planarizing the back face of the bridge chip, therebycreating an opening in the bridge chip that exposes a portion of theactive face of the base chip.

In a variation on this embodiment, the back face of the bridge chip isplanarized after the bridge chip is bonded to the base chip.

In a further variation, the back face of the bridge chip is planarizedbefore the bridge chip is bonded to the base chip.

In a further variation, the active face of the base chip communicateswith a third device through the opening in the bridge chip.

In a further variation, the active face of the base chip receives powerand ground through the opening in the bridge chip and optionallycommunicates with other devices through the opening, for instance usinga solder bump array, an optical fiber, a rigid-flex cable, a ceramicsubstrate, an organic substrate, and/or a silicon substrate.

In a further variation, the planarization of the back face of the basechip increases the compliance of the portion of the bridge chip used forproximity communication.

In further variation, a pit etched in the bridge chip facilitatesalignment with a neighboring chip for proximity communication.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an array of chips communicating using proximitycommunication in accordance with an embodiment of the present invention.

FIG. 2 illustrates the formation and scaling of a two-layer stack ofchips in a planar configuration in accordance with an embodiment of thepresent invention.

FIG. 3 illustrates a compliant bridge chip in accordance with anembodiment of the present invention.

FIG. 4 presents a flow chart illustrating the process of creating acompliant island-bridge chip in accordance with an embodiment of thepresent invention.

FIG. 5A illustrates the active face of a bridge chip with etched regionsto be removed in accordance with an embodiment of the present invention.

FIG. 5B illustrates the top view of a bridge chip after thinning inaccordance with an embodiment of the present invention.

FIG. 6A illustrates a top view of the island chip and the etched regionsof the bridge chip before thinning in accordance with an embodiment ofthe present invention.

FIG. 6B illustrates the top view of the bridge chip and island chipafter thinning in accordance with an embodiment of the presentinvention.

FIG. 6C illustrates a bottom view of the bridge chip and island chipafter thinning in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the claims.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. This includes, but is not limited to, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or any devicecapable of storing data usable by a computer system.

Proximity Communication

Proximity communication overcomes the limitations of resistive wires bycommunicating through capacitive coupling between chips that are placedface-to-face. As shown in FIG. 1, for an array of chips communicatingthrough proximity communication, the active faces 102 of the chipstypically face each other with full or partial overlap, with thecorresponding proximity communication pads 104 on opposing chips alignedboth laterally and vertically. The back faces 106 of chips are alsolabeled for clarity.

In one embodiment of the present invention, the lower chips in FIG. 1are referred to as “island chips” 108, and the upper chips are referredto as “bridge chips” 110. In such an arrangement, the bridge chips 110have less active circuitry and power consumption and serve primarily toconnect two or more island chips together.

To enhance proximity communication between neighboring island chips, theisland chips and corresponding bridge chips are precisely aligned bothlaterally as well as axially (e.g. in the z-direction) to enablereliable signal transfer between chips. Previous methods typically relyon rigid, non-compliant bridge and island chips that are difficult toalign in large arrays. Thinning the bridge chips achieves flexibilitybut is sub-optimal because the resulting chips may be difficult tohandle and may be less reliable. The present invention overcomes theseand other drawbacks by providing a structure and method of manufacturefor a bridge chip that achieves a precisely defined, predeterminedamount of compliance or flexibility.

Compliant Bridge Chips

Creating compliant bridge chips for proximity communication presents aset of difficulties. While a thin bridge chip provides compliance,bridge chips are typically conductively bonded to one or more islandchips for power. To simplify the handling of the bridge chips, bridgechips can be permanently attached to an island chip, for instance usinga flip-chip process with solder bumps that provides permanent conductivechannels between the bridge chip and the island chip. The bridge chip,now permanently attached to the first island chip and remateablyattached to a second island chip, provides interconnection between thetwo island chips.

Unfortunately, while the thinning of semiconductor wafers is a knownprocess, bonding thin chips can be challenging. For instance, siliconcircuits are routinely thinned after fabrication to reduce the substratethickness and improve electrical and thermal characteristics of thepackaged chips. While these techniques can be used to create compliantbridge chips, handling and especially flip-chip bonding the thin waferscan be substantially difficult. Such difficulty is especially common forwafers that have been thinned to the point where they exhibit somephysical compliance (i.e. flexible wafers).

Existing bonding techniques “flip-chip bond” one semiconductor chip ontoanother, and then remove the substrate of the second chip. Thistechnique enables the flip-chip bonding of chips of different materials(e.g. GaAs onto Silicon, or InP onto Silicon), but can be generalized toattach arbitrary chips. The present invention teaches a method thatallows a compliant bridge chip with arbitrary flexibility to be createdand to then be permanently attached to an island chip. Furthermore, themethod enables a “cutout” section to be defined so that flexibility canbe imparted to a pre-determined sub-sectional region of the bridge chip.Such a configuration can impart flexibility to only the desiredportion(s) of the bridge chip that extend between the correspondingislands.

FIG. 4 presents a flow chart illustrating one embodiment of the processfor creating a compliant island-bridge chip, while FIG. 2 illustrateshow a two-layer stack of chips can be formed and scaled in a planarconfiguration. The first step is to manufacture a set of island chipsand bridge chips (step 402). Next, the host island chip is prepared forsubsequent flip-chip bonding by adding micro solder-bumps 202 (andpossibly solder-bump balls, as described later) (step 404). This can beaccomplished using a photolithographic technique at the wafer level thatoperates on multiple island chips in parallel.

The bridge chip then undergoes an optional initial round of thinning,resulting in a partially-thinned bridge chip 204 (step 406). Thisoptional thinning operation achieves a predetermined amount offlexibility in the bridge chip that allows the compliant sections of thebridge chip to be formed at the wafer level. Thinning portions ofmultiple bridge chips in parallel reduces the cost associated withindividual chip thinning, while still enabling the rigid sections of theresulting thinned and sawed individual bridge chips to be easily handledand flip-chip bonded to corresponding island chips via athermo-compression bonding procedure.

After the optional thinning, the bridge chip is bonded to the hostisland chip 206 to establish mechanical and electrical contact betweenthe chips (step 408). The bridge chip can then be further thinned forcompliance 208 (step 412). FIG. 3 illustrates a compliant bridge chipand how the compliance can compensate for dynamic mechanicalperturbations and chip height differences associated with non-planarmulti-chip packages. The resulting compliant, flip-chip bondedisland-bridge chip combination can be tiled to form two-dimensionalbridge chip structures for proximity communication.

Note that the thinning of the bridge chips may be accomplished by aphysical process, a chemical process, or some optimal combination ofboth physical and chemical techniques, depending on the desired accuracyand control of the final thickness and the desired thinning rate. Notealso that the thinning of the substrate can benefit the performance ofproximity communication circuitry by reducing parasitic capacitance.

Removing Etched Portions of Bridge Chips

In one embodiment of the present invention, portions of the bridge chipwafer are identified for removal, and the area on the active face of thechip that corresponds to the areas to be removed is etched duringmanufacture. Etching can include, but is not limited to, wet etching,which removes material by immersing the wafer in a liquid bath ofchemical etchant, and dry etching, which typically removes material byexposing the wafer to a bombardment of ions. Note that only a relativelythin portion of the top of the chip might be etched (e.g. 100 microns);the etching does not need to go completely through the wafer. Then,either before or after being bonded to an island chip, the back face ofthe bridge chip is thinned. Since the etched portions of the active facehave essentially thinned the portions of the bridge chip to be removed,the subsequent thinning of the back face removes the identified portionswhile leaving the un-etched portion thin and compliant, as desired.

FIG. 5A illustrates the active face of a bridge chip with etched regions502 to be removed. The bridge chip begins as a full chip. The regionwith micro-bumps 504 indicates where the bridge chip will be bonded toan island chip. Etched pits 506 created during the manufacturing processaid in the alignment and coupling of the proximity communication pads ofthe bridge chip with the pads of neighboring island chips.

FIG. 5B illustrates the top view of a bridge chip after thinning. Theetched portions 502 of the bridge chip have been removed, leaving ahollowed-out area in the center of the bridge chip 508. Note thatthinning can occur before bonding, after bonding, or both before andafter bonding. The rigid bridge chip can be handled and bonded easilywhen the bonding is performed before major thinning.

FIGS. 6A-6C illustrate a bridge chip bonded to an island chip beforethinning. FIG. 6A illustrates a top view of the island chip and theetched regions 502 of the bridge chip before thinning. The bridge chipcan be thinned to an arbitrary thickness, depending on the specified ordesired depth of the features of the bridge chip. FIG. 6B illustratesthe top view of the bridge chip and island chip after thinning. FIG. 6Cillustrates a bottom view of the bridge chip and island chip afterthinning. Note that the hollowed-out portion of the bridge chip 508exposes a portion of the active face of the island chip.

Since the etched portions of the bridge chip to be removed are thinnerthan the area of the bridge chip that the island chip is bonded to, theprocess of thinning the bridge chip does not reach or damage the exposedactive face of the island chip. Furthermore, after thinning, the exposedactive face of the island chip can be used to attach the island chip toa communication mechanism such as an optical fiber or a rigid-flex cableor to attach the island chip to a second level package (step 414) usinga solder bump array 602 that protrudes past the bridge chip. Processinga chip in a way that adds proximity communication functionality whileexposing areas on the chip so that industry-standard processes can beused on the center of the chip to enable a wide range of functionality.Potential applications for this configuration include bonding the islandchip to a circuit board, specifying a single island chip to receiveoptical signals and distribute the data to surrounding chips usingproximity communication, and easily adding proximity communication toexisting chips without changing the manufacturing process or yield ofthe original chip. For instance, a host island chip and a bridge chipmanufactured using a process amenable to thinning could be bonded to aprocessor package.

In summary, the techniques in the present invention can be applied tomanufacture a bridge chip with a pre-determined amount of compliancethat can be bonded to an island chip that provides mechanical supportand power to the bridge chip. The compliance in the bridge chip allowsthe system to maintain a desired target separation between the chipswhile avoiding handling and reliability issues.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A system for facilitating proximity communication betweensemiconductor chips, comprising: a base chip; and a bridge chip; whereinthe base chip and the bridge chip each include an active face upon whichactive circuitry and signal pads reside, and a back face opposite theactive face; wherein the active face of the bridge chip is bonded to theactive face of the base chip; and wherein an identified portion of theactive face of the bridge chip is thinned via etching and is removed byplanarizing the back face of the bridge chip, thereby creating ahollowed-out opening in the bridge chip that exposes a portion of theactive face of the base chip.
 2. The system of claim 1, wherein the backface of the bridge chip is planarized after the bridge chip is bonded tothe base chip.
 3. The system of claim 1, wherein the back face of thebridge chip is planarized before the bridge chip is bonded to the basechip.
 4. The system of claim 1, wherein the active face of the base chipcommunicates with a third device through the opening in the bridge chip.5. The system of claim 4, wherein the active face of the base chipreceives power and ground through the opening and optionallycommunicates with other devices through the opening using: a solder bumparray; an optical fiber; and/or a rigid-flex cable; a ceramic substrate;an organic substrate; and/or a silicon substrate.
 6. The system of claim1, wherein planarizing the back face of the base chip increases thecompliance of the portion of the bridge chip used for proximitycommunication.
 7. The system of claim 6, wherein a pit etched in thebridge chip facilitates alignment with a neighboring chip for proximitycommunication.
 8. A computer system that includes: a processor coupledto a memory; a base chip; and a bridge chip; wherein the base chip andthe bridge chip each include an active face upon which active circuitryand signal pads reside, and a back face opposite the active face;wherein the active face of the bridge chip is bonded to the active faceof the base chip; and wherein an identified portion of the active faceof the bridge chip is thinned via etching and is removed by planarizingthe back face of the bridge chip, thereby creating a hollowed-outopening in the bridge chip that exposes a portion of the active face ofthe base chip.
 9. The computer system of claim 8, wherein the processoror the memory is the base chip.